In the field of surgery as well as in many other technical disciplines there is a great need for a suction system in which the vacuum or negative pressure level can be highly controlled. In ophthalmic surgery, for example, many cutting instruments draw the tissue into the cutting edges by the use of suction. Indeed, the tissue removal rate is effectively controlled by the suction effect which is related directly to the negative pressure level. Thus controlling the negative pressure level to a fine degree is highly desirable to provide to the surgeon a concomitant degree of control of the tissue removal process.
However, prior art suction systems are generally deficient in their poor control of the vacuum level. Many systems employ a pressure delivery tank in which the vacuum level is controlled by selective connection to a lower pressure source or to a selectively valved "leak" from atmospheric pressure. These systems are characteristically underdamped pressure oscillators, in that the negative pressure level often swings wildly about the desired (and often changing) level. Furthermore, purposely introducing a leak into a vacuum system is inherently wasteful, and contradicts sound engineering practice. Also, the large volume of most systems causes a delay in their response, which may lead to poor user control and overshooting of the desired vacuum level.
Some suction systems deliver vacuum to a surgical handpiece, and provide a series of leak ports adapted to be blocked or unblocked by one or more of the fingers of the hand wielding the handpiece. The surgeon may slide a finger along the row of ports to unblock ports and thus set a limited number of vacuum suction levels. This pressure control is not continuous, being limited to a few discrete levels, and even these levels can be set only within a broad range. Here again, introducing a series of leaks is inherently inefficient and wasteful.